Recently, high-density packaging and downsizing in thickness is increasing in electronic device fields such as surface mount devices (SMD) having ball grid array (BGA) or land grid array (LGA). Conventionally, a plurality of lands 100 is provided on a printed circuit board 1000 as illustrated in FIGS. 10 and 11. Then, an appropriate amount of solder 200 is supplied onto each of the lands 100, and a BGA or LGA type electronic device 300 is mounted on the solder 200. Subsequently, the solder is melted by heating, and then cooled to electrically connect the lands 100 of the printed circuit board 1000 and the terminals of the mounted device.
However, in case that the 500 or greater terminals and/or 20 mm or greater length on a side of the electronic device, the amount of warpage in the printed circuit board 1 may become significant. In addition, a fine-pitch, e.g., 0.4 mm or less, between terminals of the electronic device has been increasing. Therefore, in order to maintain connectivity in an initial stage, control of the warpage amount of the printed circuit board has generally become tighter and/or stricter. FIG. 12A illustrates a printed circuit board 1000 which has warped convexly, i.e. warped upward, whereas FIG. 12B illustrates a printed circuit board 1000 which has warped concavely, i.e. warped downward. If a printed circuit board 1000 warps, a solder bump and a terminal of the device may fail to contact each other as illustrated in part X of FIG. 12A and part Y of FIG. 12B, thus causing a so-called open-circuit fault.
On the other hand, in case of increasing the supply amount of solder, open-circuit faults may be avoided. Instead, adjacent solder bumps in a molten state, however, may have contact with each other as illustrated in part Y of FIG. 12A, in part X of FIG. 12B, in FIG. 13A and FIG. 13B, thus causing a so-called bridging (short circuit) fault.
In FIG. 13, since a surface mounted device of BGA or LGA structure has fine-pitch terminals on the bottom, a bridging fault between adjacent terminals may be caused in case of over-supplied solder.
As seen in this manner, the above-mentioned faults may depend on a warpage shape of the printed circuit board. Therefore, connection reliability between the printed circuit board and the electronic device is difficult is difficult to maintain and/or increase by adjusting the supply amount of solder.
Japanese Patent Application Publication No. 10-294554, for example, discloses a printed circuit board on which at least two parallel grooves are provided between adjacent lands. The parallel grooves extend perpendicular to an array of the adjacent lands. However, according to JP-A-10-294554, an extra step of forming the parallel grooves on the printed circuit board other than conductor patterns is needed, thus incurring additional costs and time in board fabrication.